Preheat the oven to 350°F (175°C).

In a skillet, sauté the onion and garlic until translucent.

Add the sliced yellow squash and cook until just tender, about 5 minutes.

In a mixing bowl, combine the cooked squash mixture, sour cream, cheddar cheese, beaten egg, salt, pepper, and thyme.

Transfer the mixture to a greased baking dish.

In a separate bowl, mix the bread crumbs with melted butter and sprinkle over the casserole.

Bake for 25-30 minutes or until the top is golden brown.